Step 1: Article 23 of the Constitution prohibits traffic in human beings, begar, and other similar forms of forced labour, and the question asks which case stretched what forced labour actually covers.
Step 2: Before this case, forced labour was understood mainly as physical compulsion, someone being made to work against their will through threats or coercion.
Step 3: In People's Union for Democratic Rights v. Union of India, the Supreme Court examined workers on Asian Games construction sites who were paid less than the minimum wage, and held that being forced by poverty to accept sub-minimum wages is itself a form of forced labour under Article 23, since the worker has no real choice.
Step 4: That shift, from requiring physical coercion to also recognising economic compulsion, is what makes this the case that expanded Article 23, unlike Vishaka, M.C. Mehta v. State of Tamil Nadu, or Glaxo Laboratories, which deal with sexual harassment, child labour under Article 24, and ordinary labour court disputes respectively.
\[ \boxed{\text{PUDR v. Union of India}} \]